This present invention relates to bicycle rims, and more particularly, to a bicycle rim having a brake track.
Bicycle wheels and rims have been in use for well over a century, Historically, most bicycle rims have been made of steel or aluminum. However, in the past few decades, some bicycle rim manufacturers have begun to produce bicycle rims from other materials, such as lightweight fiber-reinforced plastics (FRPs), including carbon-fiber, fiberglass, and nylon fibers, that are mixed in a resin such as epoxy-based resins, phenolic-based resins and/or ester-based resins. Of these composite rims, some have been made entirely of FRP composite materials (“full composite wheels”), whereas others have incorporated components of different materials in addition to FRP composites “multi-component rims”).
Carbon fiber rim brake surfaces have been difficult to design. Early carbon rims used the molded surface of the rim as a brake track or braking surface, which was generally troublesome since the as-molded rims typically had a thin layer of mold release embedded in the surface, the presence of which was not suitable for a braking surface since mold release materials yield a low coefficient of friction. The mold release layer is used to remove the rim from the mold during manufacturing. After a short period of brake use, the mold release and outer layer of epoxy on the rim wore away, exposing the vulnerable carbon fiber beneath. Carbon surfaces are relatively poor at resisting wear and enhancing frictional performance. Thus, carbon fiber surfaces are generally unsuitable as brake tracks for FRP rims.
Later rims use fiberglass, quarts fiber, or Kevlar fibers in the brake track. All these materials have better wear characteristics than carbon and provide varying degrees of improved braking performance. Kevlar is quite good as a frictional material but is subject to moisture wicking which can lead to delamination from the rim. Kevlar has only average wear characteristics in the field once exposed to wet conditions or abrasive road grit which can contain concrete or asphalt particles, as well as dirt and sand which can embed in the brake pad causing extreme and rapid rim wear. Fiberglass and quartz are average in frictional performance and in wear performance, while also being inexpensive, but these materials can be heavy without providing additional strength or toughness to the product.
One approach to provide a good brake track involves removal of the epoxy and mold release from the brake track area. This is accomplished through a mechanical abrasion of the rim surface that both removes the mold release and improves parallelism or alignment of the brake tracks. Later generations of rims have used a veil of silicon fibers mixed with glass. The silicon fibers are harder than glass and offer significantly improved wear performance as well as brake ‘feel’ but are very expensive, difficult to work with due to high fiber stiffness and add considerable manufacturing cost. While modern aircraft and auto racing brakes are currently being made from silicon carbide fibers, these materials are currently quite expensive, hard to process and are difficult to form into smaller radii, especially in a manufacturing setting. This lack of pliability limits their application to rim design and potentially prohibits use for certain complex rim shapes.
Some rim manufacturers have experimented with post-applied ‘ceramic’ brake track coatings. These range from painted-on applications that are heat cured, to plasma-sprayed coatings. All of these coatings offer aggressive frictional surfaces but suffer from being very brittle, heavy and generally requiring high temperature application procedures that can damage the molded rim. These coatings also suffer from geometric application issues, since being post-applied, the brake surfaces are inherently imperfect and non-parallel due to lack of geometric control.
There is a demand, therefore, to provide a bicycle, rim with a durable and cost-effective brake track with excellent braking characteristics. The invention satisfies the demand with minimal effect on rim weight and tire-mounting effort.